U.S. Republican presidential nominee Donald Trump tweeted Tuesday that Barack Obama "will go down as perhaps the worst president in the history of the United States" after Obama called Trump "unfit" to lead.

"I think the Republican nominee is unfit to serve as president. I said that last week and he keeps on proving it," Obama said Tuesday at the White House.

But a former U.S. Marine attending a Trump rally Tuesday in Virginia told VOA he thinks Trump will be "a great commander-in-chief." A female Trump fan called him "the real deal."

The outspoken Trump added another controversy to his growing list Tuesday when he accepted a Purple Heart medal from a retired lieutenant colonel before the rally in Ashburn, Virginia. Trump said the former soldier told him the gesture was intended to show confidence in him.

The Purple Heart is awarded to a U.S. serviceman or woman wounded in combat, or posthumously awarded to someone killed in battle. It is a sacred U.S. military tradition.

Republican U.S. Presidential nominee Donald Trump talks to Lt. Col. Louis Dorfman, who gave Trump his Purple Heart, during a campaign event at Briar Woods High School in Ashburn, Virginia, Aug. 2, 2016.

Trump, who says he regrets never serving in the military, said he was honored by the soldier's gift. The audience chuckled when he said he had "always wanted to get the Purple Heart. This was much easier."

But a spokesman for the Military Order of the Purple Heart organization, John Bircher, was not amused.

"It is absolutely horrible for anyone to wear or have the Purple Heart medal who is not entitled to it," Bircher told CBS News.

"Donald Trump did not get the Purple Heart and there's no 'easy way' to get it. I don't think he has any clue as to the meaning of the Purple Heart medal."

Khan backlash

Obama, who has endorsed Democrat Hillary Clinton as his successor when he leaves office in January, challenged Republicans to repudiate Trump, saying his complaints against Khizr Khan and his wife, Ghazala, after they endorsed Clinton at last week's Democratic National Convention were offensive. The Muslim American couple's son was killed fighting for the U.S. in Iraq in 2004.

Khizr and Ghazala Khan, the parents of an Army captain killed in Iraq, speak with VOA in Washington, D.C., August 1, 2016. (Brian Allen/VOA)

"The notion that he would attack a Gold Star family that had made such extraordinary sacrifices on behalf of our country," Obama said of Trump. "The fact that he doesn't appear to have basic knowledge about critical issues, in Europe, in the Middle East, in Asia, means that he's woefully unprepared to do this job."

Gold Star families in the U.S. are those that have lost relatives while fighting in the U.S. armed services.

'Failed leadership'

In response, Trump accused Obama of "failed leadership." Trump said that Obama and Clinton, his first-term secretary of state, had "destabilized the Middle East, handed Iraq, Libya and Syria" to Islamic State jihadists. The Republican nominee said they also had sent America's "best jobs overseas to appease their global interests."

"We need change now," Trump concluded.

In a highly unusual attack on his possible replacement, Obama questioned why Republicans continue to endorse Trump, even as numerous Republican lawmakers have condemned his comments about the Khans, sometimes criticizing Trump by name and other times denouncing his comments but not mentioning him by name.

Trump said the the Khans had "viciously attacked" him at the Democratic convention and then complained that they continued to assail his candidacy in a series of television interviews in recent days. Khizr Khan said Trump had made no sacrifice for the U.S. as great as his son, Humayon Khan, killed by a suicide bomber while protecting other soldiers.

Trump, in one interview last weekend, said he had made sacrifices for the United States by creating "thousands of jobs" and providing workers benefits to improve their lives.

FILE- This June 3, 2016, file photo shows Sen. John McCain, R-Ariz., delivering a speech. Senate Republicans running for re-election weighed in one after another on Aug. 1, 2016, to condemn Republican presidential candidate Donald Trump’s repeated attacks on the parents of slain U.S. Army Capt. Humayun Khan, with former prisoner of war Sen. John McCain of Arizona leading the charge.

Arizona Republican Senator John McCain, who lost to Obama in the 2008 presidential contest, said he deeply disagrees with Trump's suggestions that Muslims like the Khans should not be allowed in the country.

"It is time for Donald Trump to set the example for our country and the future of the Republican Party," McCain said. "While our party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us."

McCain was a Navy combat pilot during the Vietnam War and spent more than five years as a prisoner of war. Earlier in the campaign, Trump denigrated McCain's status as a war hero, saying he prefers people who do not get captured.

Iowa Senator Chuck Grassley said the families of military service members are owed "the utmost respect."

"Mr. Trump’s comments are not in line with my own beliefs about how the members of the military and their families should be treated, and respect for the people who serve our country is something both presidential campaigns could use more of," he said.

A group of 23 families of fallen soldiers released a letter to Trump calling his comments "repugnant and personally offensive."

"When you question a mother's pain, by implying that her religion, not her grief, kept her from addressing an arena of people (at the Democratic convention), you are attacking us. When you say your job building buildings is akin to our sacrifice, you are attacking our sacrifice," they wrote. "This goes beyond politics. It is about a sense of decency. That kind decency you mock as 'political correctness.'"

The group Veterans of Foreign Wars said Trump's comments are "out of bounds."

"Election year or not, the VFW will not tolerate anyone berating a Gold Star family member for exercising his or her right of speech or expression," VFW national commander Brian Duffy said.

Righting the wrong

Khizr Khan was asked during an interview Monday with VOA's Urdu service whether he thought anything useful will come from his feud with Trump.

"It really has come out, It really, really has come out that a significant larger number of Republicans are asking him to tone down, change those derogatory remarks about minorities, not only just Muslims but other minorities," Khan said.

He further expressed worry about the consequences if Trump becomes the commander-in-chief of the U.S. military and wondered whether U.S. forces would follow his orders.

Trump's running mate, Indiana Governor Mike Pence, called Humayon Khan a hero and blamed the policies of President Barack Obama and Clinton for allowing Islamic State to overrun a "once stable Middle East."

Trump wrote in a Twitter post Monday that the debate is not about the Khans, but about "radical Islamic terrorism."

What's next

Much of Trump's speech Tuesday in Virginia focused on the economy and terrorism.

He said it would be "a good thing" to partner with Russia and others and "knock the hell" out of Islamic State.

He also said electing Clinton as president would mean four more years of Obama's policies. If that happens, Trump says, the country would be "finished."

Clinton suspended her campaign Tuesday to attend the funeral in Rhode Island of her friend Mark Weiner, a Democratic party donor and fundraiser. Weiner died of cancer last week at 62.

Clinton plans to resume her campaign Wednesday in Commerce City, Colorado. Trump is scheduled to speak in Daytona Beach and Jacksonville, Florida.